Oil shale is the colloquial term for a wide variety of laminated sedimentary rocks containing organic matter that can be released predominantly only by destructive distillation. While some removal of organic matter by solvents is possible, the amount so removed is quite small unless supercritical extractions are employed. This characteristic permits clear distinction from tar sands which are rock or sand formations actually impregnated with oil.
While oil shales have been utilized as a source of fuel for centuries, such uses have generally been small, and the great potential for the huge deposits in various locations around the world remains to be unlocked on a feasible commercial scale.
Oil shales generally contain over one-third mineral matter. The organic portion, a mixture of complex chemical compounds, has been termed "kerogen". Kerogen is simply a generic name for the organic material found in such circumstances, but it is not a definite material since kerogen compositions differ when derived from differing shales.
Destructive pyrolysis of crushed oil shale yields shale oil, a dark, fairly viscous organic liquid. Under the pyrolysis conditions commonly employed, a disproportionation of carbon and hydrogen structures equivalent to internal hydrogenation-dehydrogenation is believed to occur. A large percentage of the kerogen converts to a liquid (the shale oil), some to light gases, and the rest remains as a carbon-rich residue on the inorganic matrix.
Shale oil in some respects may be considered as intermediate in composition between petroleum and coal tar, comparing for example the H:C atomic ratio of about 18:10 for light crude oil, about 15:10 for shale oil, and about 13:10 for coal carbonization products. Shale oil, of course, is not "crude oil" in the strict sense, though refining steps for the shale oil are similar to the refining steps applied to petroleum crudes.
Retorting of ground oil shale fines using heat obtained from radiant solar energy through mirror arrangements has heen employed in recent years experimentally. The basic apparatus arrangement so far employed has comprised a large mirror collector producing a converging beam of light directed to and through a transparent "window" aperture opening into a retort chamber to provide a contacting zone wherein the oil shale fines are exposed to the concentrated thermal energy at the focal zone of the solar beam, thus resulting in heating of the oil shale.
The oil shale takes up heat from the concentrated thermal energy, converting to a large extent the kerogen, and producing oil vapors which then are drawn off into a collector system. The spent shale is transferred to another collection chamber. Depending on the time of exposure and the amount of thermal energy employed per unit measure of oil shale, there may be varying amounts of residual carbon or carbonaceous residue left in the oil shale.
Problems encountered in solar retorting of oil shale include keeping the transparent window clean and free of energy-absorbing energy-robbing deposits. Any reduction of effective energy transfer results in immediate dropoff in heat in the oil shale, and loss of process effectiveness and efficiency. Further problems have been encountered in heating large quantities of ground oil shale fines, not just simply a thin layer on the surface of a deep bed which is inefficient and may result in undesirable decomposition products and inadequate conversions of kerogen to shale oil. Other problems have been encountered in avoiding the absorption of radiant energy by smoke and/or mist arising from the oil shale being retorted, which results in further heating and degrading of the smoke and/or oil mist rather than desired heating of oil shale itself.